Capacitors for medical devices

ABSTRACT

The invention is directed to designs for capacitors of implantable medical devices (IMDs) such as implantable defibrillators, implantable cardioverter-defibrillators, implantable pacemaker-cardioverter-defibrillators, and the like. The capacitor designs can reduce capacitor volume significantly and may also improve charge holding capacity relative to conventional capacitor designs. Moreover, since capacitors typically comprise a significant portion of the volume of an IMD, significant reductions in capacitor volume can likewise significantly reduce the size of the IMD.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 10/774,210filed Feb. 6, 2004 now U.S. Pat. No. 7,555,339 and entitled “Capacitorsfor Medical Devices”.

TECHNICAL FIELD

The invention relates to medical devices and, more particularly, to highvoltage capacitors for storing energy in and delivering therapy frommedical devices.

BACKGROUND

High voltage capacitors are commonly used in various medical devices inorder to store charge so that electrical stimulation therapy (e.g.,defibrillation and/or cardioversion) can be delivered to a patient. Forexample, defibrillators commonly make use of one or more high voltagecapacitors to store charge prior to delivery of high voltagedefibrillation therapy to a patient. Defibrillation electrical therapycan be delivered to depolarize the patient's heart and thereby overcomean episode of a potentially lethal arrhythmia (e.g., ventricularfibrillation).

Many different medical devices have been designed with defibrillationcapabilities. Examples include the automatic external defibrillator(AED), implantable cardioverter-defibrillators (ICD) and the like.Capacitors are commonly used in such medical devices to store charge fordelivery of cardiac defibrillation therapy and/or cardioversion therapy,and the like. Other types of medical devices, including medical devicesnot yet developed, may also implement high voltage capacitors for theseor other applications.

Medical devices that provide defibrillation therapy typically include atleast the following primary components: a power source, one or morecapacitors, at least a pair of cardiac electrodes, and circuitry tocontrol delivery of cardiac therapy. The capacitor(s) typically consumea significant portion of the volume of a medical device, particularly anIMD. Improved medical device capacitor designs are highly desirable,particularly for an ICD, to achieve size reductions strongly associatedwith patient acceptance and comfort, among other reasons.

SUMMARY

In general, the present invention provides improved high energy densitycapacitor apparatus and methods of fabrication therefore. Capacitorsdesigned according to the present invention provide reduced capacitorvolume as compared to the prior art and may also improve the chargestorage capacity relative to prior art capacitor designs. Moreover,since capacitors typically comprise a significant portion of the volumeof an ICD, reductions in capacitor volume likewise significantlyprovides an opportunity to further reduce the overall size of medicaldevices incorporating them.

The present invention provides a medical device including circuitry tocontrol delivery of electrical therapy to a patient and at least onecapacitor to store charge for use in the delivery of the electricaltherapy. The capacitor may comprise a first anode member composed ofpowered metal that is fabricated in situ within a first encasementmember, or shell. During such processing a desired quantity of apowdered metal such as tantalum is pressed, sintered and anodized, or“formed” (i.e., an oxide is grown on the surfaces of the member in thepresence of a formation electrolyte) as is known in the art.Alternatively, the first anode member can be processed prior to placingit in the first encasement shell. According to the present invention asecond anode member resides in a second encasement shell (again withprocessing occurring either in situ or prior to being placed into thesecond encasement shell). A cathode member is then sandwiched betweenthe first and second anodes (within the first and second encasementshells) with, as necessary, suitable separator material disposedtherebetween.

For all the embodiments of the present invention described herein,wherein more than one cathode member is desirable, a ratio of 2:1 (twoanode members for each cathode member) is employed. Also, while notalways specified herein, as is known in the art the anodes are typicallyelectrically insulated from each cathode member with a thin separatorsheet and the interior volume of the capacitor cell is filled with asuitable working electrolyte. While not specified or depicted hereineach capacitor cell typically requires at least one electricalfeedthrough coupled to either the pair (or multiple pairs) of anodes oreach cathode and a fill port for admitting liquid electrolyte.

In another embodiment, the invention provides a medical device includingcircuitry to control delivery of electrical therapy to a patient and acapacitor to store charge for use in the delivery of electrical therapy.According to this embodiment, a capacitor has a single anode pressed,sintered and formed within a first electrically conductive encasementshell (or the single anode may be constructed and electrically coupledto the first encasement shell); a cathode constructed within (or on asurface portion of a second shell) or constructed and subsequentlyelectrically connected to the second electrically conductive encasementshell; and an insulative material disposed at the interface of the firstand second encasement shells to electrically isolate the shells (andthus the anode and the cathode) from each other.

In another embodiment, the present invention provides a capacitorcomprising an anode formed within a first encasement shell, a cathodeforming a second encasement shell, and electrically insulative materialdisposed at the interface of the first and second encasement shells toelectrically isolate the anode from the cathode.

In another embodiment, the present invention provides a methodcomprising processing (i.e., pressing, sintering and forming) a quantityof tantalum powder into a first and a second tantalum encasement shellto define first and second anodes, applying a thin titanium substratemember with ruthenium oxide disposed on both sides to form a cathode,wrapping a non-conductive separator sheet around the cathode, andsandwiching the cathode between the first and second anodes to fabricatea single capacitor cell.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic representation of an exemplary medical device suchas an ICD, which can implement a capacitor design in accordance theinvention.

FIG. 2 is a functional block diagram of an IMD such as that shown inFIG. 1 comprising a pacemaker that includes both pacing anddefibrillation functionality.

FIG. 3 is an exploded cross-sectional view of a capacitor according toan embodiment of the invention.

FIG. 4 is an assembled cross-sectional view of the capacitor illustratedin FIG. 3.

FIG. 5 is a flow diagram illustrating a process for fabricating thecapacitor illustrated in FIGS. 3 and 4.

FIG. 6 is an exploded cross-sectional view of another capacitoraccording to another embodiment of the invention.

FIG. 7 is an assembled cross-sectional view of the capacitor illustratedin FIG. 6.

FIG. 8 is a flow diagram illustrating a process for creating thecapacitor illustrated in FIGS. 6 and 7.

DETAILED DESCRIPTION

The invention is directed to capacitor designs for capacitors of medicaldevices such as implantable cardioverter-defibrillators (ICDs) and tonon-implantable devices such as manual or automated externaldefibrillators (AEDs). In the description that follows, primarily forconvenience such medical devices are referred to as ICDs with theunderstanding that any cardiac defibrillation device is also coveredthereby, including manual defibrillators and AEDs. Also, while mostreferences to many of the details of the invention are provided in thecontext of a capacitor used to store charge for deliveringdefibrillation electrical therapy to a patient. The capacitor designsdescribed herein, however, may find use in a wide variety of otherdevice applications, including non-medical applications wherein highenergy density and compact cell size are significant design constraints.That is, the present invention offers significant reduction in capacitorcell volume, and thus, may be most useful in applications where size isa primary concern.

FIG. 1 is a schematic representation of an exemplary IMD 10 implementingone or more aspects of the invention. In this case, IMD 10 is a threechannel pacing device shown in conjunction with a human heart 12. IMD 10also includes cardioversion and defibrillation functionality and may bereferred to as a pacemaker-cardioverter-defibrillator. IMD 10 includes acapacitor that, as outlined in greater detail below, can achieve reducedvolume relative to various conventional capacitors. As mentioned above,the invention may also find application in numerous other types of IMDsor external medical devices.

As shown in FIG. 1, IMD 10 includes left ventricular (LV) coronary sinuslead 14, which is passed through a vein into the right atrium of heart12, into the coronary sinus and then inferiorly in the great cardiacvein and cardiac veins extending from the great vein to extend thedistal ring pace/sense electrodes alongside the LV chamber. The distalend of LV coronary sinus lead 14 positions ring electrodes optimallywith respect to the adjacent wall of the left ventricle. Rightventricular (RV) lead 26 is typically passed through either the superioror inferior vena cava that leads into the right atrium and feeds intothe right ventricle where its distal ring and tip pace/sense electrodesare fixed in place in the apex, RV outflow tract or in theinterventricular septum, or the like. Right atrium (RA) lead 15 ispositioned within the RA chamber, with distal end of RA lead 15positioning ring electrodes optimally with respect to the adjacent wallof the right atrium or positioned within the RA appendage. The differentleads may include electrodes for pacing and also high voltage coilelectrodes for cardioversion or defibrillation therapy.

FIG. 2 is a functional block diagram of an embodiment of IMD 10, such asthat shown in FIG. 1 in which IMD 10 comprises a pacemaker that includeboth pacing and defibrillation functionality. As shown in FIG. 2, IMD 10includes an electrode system. Electrode 110 in FIG. 2 includes theuninsulated portion of housing 42 of IMD 10. Electrodes 110,111,113,115are positioned along one or more of leads 14,15,26 and are coupled tohigh voltage output circuit 117, which includes high voltage switchescontrolled by cardioversion/defibrillation (CV/defib) control logic 119via control bus 121. Switches disposed within circuit 117 determinewhich electrodes are employed and which electrodes are coupled to thepositive and negative terminals of the capacitor bank (which includescapacitors 123,125) during delivery of defibrillation pulses.

Capacitors 123,125 may have reduced size relative to conventionalcapacitors used in IMDs. In particular, a number of capacitor featuresare described in greater detail below, which can significantly reducethe volume associated with capacitors 123,125. Moreover, sincecapacitors 123,125 comprise a significant portion of the volume of IMD10, reductions in capacitor volume can likewise significantly reduce thesize of IMD 10.

Electrodes 20,22 may be located on or in left ventricle 24 of thepatient and are coupled to amplifier 112, which may comprise anautomatic gain controlled amplifier providing an adjustable sensingthreshold as a function of the measured R-wave amplitude. For example,electrodes 20,22 may be positioned proximate to distal end leftventricular (LV) coronary sinus lead 14 (FIG. 1). A signal is generatedon LV out line 114 whenever the signal sensed between electrodes 20 and22 exceeds the present sensing threshold.

Electrodes 30,32 are located on or in right ventricle 28 of the patientand are coupled to amplifier 116. Electrodes 180,182 conceptuallyrepresent electrodes located at any desired location within or inproximity to heart 12. Electrodes 180,182 are coupled to amplifier 184.In one example, electrodes 180,182 may be positioned proximate to distalend of right atrium (RA) lead 15 (FIG. 1). However, in otherembodiments, electrodes 180,182 may correspond to any other desiredlocation. In many multi-site embodiments, pairs of electrodes areprovided at a number of locations. In other embodiments, a number ofpairs of electrodes may be used specifically for one chamber of theheart. In general, however, any number of pairs of electrodes may bedeployed in a variety of locations around the heart. The invention,however, can be used in systems having any number of leads and anynumber of electrodes.

Switch matrix 120 is used to select which of the available electrodesare coupled to wide band (0.5-200 Hz) amplifier 122 for use in digitalsignal analysis. Selection of electrodes is controlled by microprocessor124 via data/address bus 126, and the selections may be varied asdesired. Signals from the electrodes selected for coupling to band passamplifier 122 are provided to multiplexer 128, and thereafter convertedto multi-bit digital signals by A/D converter 130, for storage in randomaccess memory 132 under control of direct memory access circuit 134.Microprocessor 124 may employ digital signal analysis techniques tocharacterize the digitized signals stored in random access memory 132 torecognize and classify the patient's heart rhythm.

The remainder of the circuitry may be dedicated to cardiac pacing,cardioversion and defibrillation therapies in accordance with one ormore embodiments of the invention. Pacer timing/control circuitry 136includes programmable digital counters, which control the basic timeintervals associated with modes of pacing. Circuitry 136 also controlsescape intervals associated with pacing. Circuitry 136 also determinesthe amplitude of the cardiac pacing pulses under control ofmicroprocessor 124.

During pacing, escape interval counters within pacer timing/controlcircuitry 136 may be reset upon sensing of R-waves as indicated by asignals on lines 114 and 118. In accordance with the selected mode ofpacing, pacer timing/control circuitry 136 triggers generation of pacingpulses by pacer output circuitry 138,140 and 190 which are coupled toelectrodes 20,22,30,32,180,182. Escape interval counters may also bereset on generation of pacing pulses and thereby control the basictiming of cardiac pacing functions. The durations of the intervalsdefined by escape interval timers are determined by microprocessor 124via data/address bus 126.

In the event that generation of a cardioversion or defibrillation pulseis required, microprocessor 124 may employ an escape interval counter tocontrol timing of such cardioversion and defibrillation pulses, as wellas associated refractory periods. In response to the detection of atrialor ventricular fibrillation or tachyarrhythmia requiring a cardioversionpulse, microprocessor 124 activates cardioversion/defibrillation controlcircuitry 119, which initiates charging of the high voltage capacitors123,125 via charging circuit 127, under the control of high voltagecharging control line 129. Again, capacitors 123,125 make use ofcapacitor designs described in greater detail below.

The voltage on the high voltage capacitors 123,125 is monitored via VCAPline 131, which is passed through multiplexer 128 and in response toreaching a predetermined value set by microprocessor 124, results ingeneration of a logic signal on Cap Full (CF) line 133 to terminatecharging. Thereafter, timing of the delivery of the defibrillation orcardioversion pulse is controlled by pacer timing/control circuitry 136.Following delivery of the fibrillation or other arrhythmia therapy,microprocessor 124 returns the device to cardiac pacing mode and awaitsthe next successive interrupt due to pacing or the occurrence of asensed atrial or ventricular depolarization.

Any cardioversion or defibrillation pulse control circuitry may be usedin conjunction with various embodiments of the invention. By way ofexample, delivery of cardioversion or defibrillation pulses can beaccomplished by output circuit 117 under the control of controlcircuitry 119 via control bus 121. Output circuit 117 determines whethera monophasic or biphasic pulse is delivered, the polarity of theelectrodes, and which electrodes are involved in delivery of the pulse.Output circuit 117 also includes high voltage switches, which controlwhether electrodes are coupled together during delivery of the pulse.Alternatively, electrodes intended to be coupled together during thepulse may simply be permanently coupled to one another, either exteriorto or interior of the device housing, and polarity may be pre-set.

The embodiment shown in FIG. 2 is merely exemplary. For example, theembodiment shown in FIG. 2 may be modified to include additionalfeatures, or may be adapted to other embodiments. In particular, theembodiment in FIG. 2 may be modified for an implanted medical devicehaving electrodes mounted on any number of leads not shown in FIG. 1, ormay not include one or more of the leads shown in FIG. 1. Suchelectrodes may be coupled to a P-wave amplifier (not shown in FIG. 2)that, like amplifiers 112 an 116, provides an adjustable sensingthreshold as a function of a measured P-wave amplitude. The embodimentshown in FIG. 2 may further be modified to detect activity in or nearthe left atrium of the patient and may include physiologic transducers(e.g., mechanical, metabolic and/or electrically-based sensors).

Microprocessor 124 performs mathematical calculations to carry outarrhythmia detection and therapy algorithms known in the art of cardiacpacing. The invention may find wide application to any form ofimplantable electrical device or possibly external medical devices thatmake use of high voltage capacitors.

In accordance with the invention, IMD 10 makes use of capacitors thathave reduced volume relative to conventional capacitors. In thedescription that follows, capacitors are described as including one ormore features that can help achieve such volume reduction. Thecapacitors described with reference to FIGS. 3-4 and 6-7, for example,may be implemented as capacitors 123 and 125 of IMD 10 described above,or may find use in a wide variety of other types of medical devices. Theinvention may be most useful in applications where size is a concern,although the invention is not necessarily limited in that respect.

FIG. 3 is an exploded cross-sectional view of a capacitor 50, which maycorrespond to either of capacitors 123, 125 (FIG. 2). FIG. 4 is anassembled cross-sectional view of capacitor 50. Capacitor 50 comprises asandwiched construction case-positive design in which two anodes 51,52sandwich a cathode 53. In particular, following construction andassembly (as shown in FIG. 4), capacitor 50 comprises a first anode 51constructed within a first encasement shell 55 and a second anode 52constructed within a second encasement shell 52. The anodes 51,52 may bein intimate contact, and therefore electrical, with first and secondencasement shells 55,56, and the encasement shells 55,56 may or may notform part of the anodes 51,52. For example, anodes 51,52 can be formedby pressing tantalum powder into encasement shells 55,56. The encasementshells 55,56 may comprise tantalum encasement shells having U-shapedcross sections of different depths. Optionally, a press, multi-facetedmold, and/or a drill may be used to form tunnels, bores and/or holes 54in anodes 51,52 during construction, which can help heat dissipationduring formation and may also reduce equivalent series resistance (ESR)during use of capacitor 50. As is known in the art, following suchprocessing an oxide layer is generated during formation when anodes51,52 are immersed in a pool of formation electrolyte while electricalpotential is applied until a desired formation voltage (and oxidethickness) is achieved. In the case anodes 51,52 are formed from puretantalum powder, a layer of tantalum pentoxide is formed to a desiredvoltage (e.g., 175 to over 200 volts).

Cathode 53 is sandwiched between first and second anodes 51,52 withinfirst and second encasement shells 55,56. In addition, separatormaterial 61,62 is sandwiched between the first and second anodes 51,52within the first and second encasement shells 55,56 in order toelectrically separate cathode 53 from anodes 51,52. The separatormaterial can comprise cellulose material such as Kraft paper and thelike but, in general, the separator material needs to allow iontransport between the anodes 51,52 and cathode 53.

In accordance with the invention, the capacitor construction illustratedin FIGS. 3 and 4 may allow for significant reductions in non-chargestoring material such as separator materials 61,62 relative toconventional capacitor designs. In particular, the capacitorconstruction illustrated in FIGS. 3 and 4 may allow for energy storingcomponents such as anodes 51,52 to comprise greater than 70 percent,greater than 80 percent, or even greater than 85 percent of the volumeof capacitor 50. In other words, the amount of material forming anodes51,52 can form an increased percentage of the total volume of capacitor50. Accordingly, relative to conventional capacitor designs that areless efficient, capacitor 50 may have reduced volume, or may achieveincreased energy storage capacity with a volume similar to that of suchconventional designs.

Electrical contact pins may be formed in capacitor 50 to facilitateelectrical connection to anodes 51,52 and cathode 53. For example, afirst electrical anode pin 65 can be formed through first encasementshell 55 for electrical contact with first anode 51, and a secondelectrical anode pin 66 can be formed through the second encasementshell 56 for electrical contact with second anode 52. In order tofacilitate electrical contact to cathode 53, a feedthrough element 67can be formed through one of the encasement shells (in this case shell55). The feedthrough element 67 may include a cathode pin 68 within aninsulative material 69 for electrical contact to cathode 53. Insulativematerial 69 may comprise glass, or some other suitable insulator.Following assembly, cathode pin 68 may electrically couple to cathode 53and provide external electrical access to cathode 53 as illustrated inFIG. 4.

In order to simplify construction of capacitor 50, first encasementshell 55 can be made deeper than the second encasement shell 56.Accordingly, when cathode 53 is sandwiched between first and secondanodes 51,52 within first and second encasement shells 55,56, the firstencasement shell 55 overlaps second anode 52 to abut against secondencasement shell 56. In that case, feedthrough element 67 can be formedthrough first encasement shell 55 in order to achieve proper placementof cathode pin 68 relative to cathode 53, following assembly ofcapacitor 50.

In some cases, anodes 51,52 can provide structural support to capacitor50. By providing structural support via anodes 51,52, the mass ofencasement shells 55,56 may be reduced, which can reduce the size ofimprove capacitor 50. In some embodiments, anodes 51,52 can provide morestructural support to capacitor 50 than encasement shells 55,56. Inorder to improve such structural support of anodes 51,52, one or morestructural enhancing elements can be formed within anodes 51,52. Forexample, the structural enhancing elements may comprise electrode wiresformed within anodes 51,52, or any other structural enhancing elementthat would not significantly undermine the charge holding capacity ofanodes 51,52.

For a wet-type capacitor 50 the enclosure is filled with a workingelectrolyte (not shown) that typically differs from the formationelectrolyte. One fairly common working electrolyte comprises a sulfuricacid/glycol base electrolyte. In addition, if desired, a substantiallynon-permeable film (not shown) can be formed over capacitor 50, e.g., toimprove a hermetic barrier to the interior of capacitor 50. In thatcase, anode pins 65, 66 and cathode pin 68 may pass through thesubstantially non-permeable film. Alternatively, capacitor 50 may bepackaged in a foil pack, an insulated metal case, or any other suitablepackaging material.

FIG. 5 is a flow diagram illustrating some of the steps for a processfor creating capacitor 50. As shown in FIG. 5, a tantalum powder ispressed into tantalum encasement shells 55,56 to define a pair of anodeslugs 51,52 (71). In some cases, the press used to create anodes 51,52creates a series of tunnels or holes in the anodes in order to aid inheat dissipation during the forming process and reduce ESR duringoperation. Optionally, one or more structural enhancing materials may bedispersed throughout the metallic powder prior to pressing the anode.Such materials are known as binders or binding agents and they are usedprimarily to provide increased structural integrity and to provide achanging density “gradient” to interior portions of anodes 51,52 duringpressing. Such material can be used to control expansion, shrinkage orshape deformation. Such materials are preferably completely removedafter pressing (prior to, during or after sintering). Thus, a suitablebinder can be susceptible of complete removal simply as a result of thehigh temperature, high pressure sintering processing. In addition to orin lieu of the foregoing suitable material can be susceptible ofcompletely dissolving in a fluid bath. The anodes are sintered prior toforming the tantalum pentoxide film over surfaces 57, 58. A tantalumpentoxide film is then formed on anodes 51,52, e.g., over surfaces 57,58 to define the anode dielectric for anodes 51,52 (72). Although aspecific sintering step is not depicted in FIG. 5, the present inventiondoes not depend on a particular mode of sintering and those of skill inthe art will apply appropriate temperatures and pressures to suitablysinter an anode slug.

A cathode substrate is coated on opposing sides with hydrous rutheniumoxide, activated carbon, or other suitable cathode-active material(s) toform cathode 53 (73). In the illustrated and described embodiments, thecathode substrate comprises a titanium member. Said titanium member maybe substantially flat, perforated and/or surfaced with minute featuresto enhance the bond with the cathode-active material. Cathode 53 is thensandwiched between anodes 51,52 to define capacitor 50 (74). Separatormaterial 61,62 may be provided to electrically isolate cathode 53 fromanodes 51,52 in the final construction of capacitor 50. Of course, whileseparator material 61,62 is depicted as individual members, a singlesheet portion (or more than one sheet) of separator material may be usedto essentially wrap and thereby insulate the cathode 53. In the finalconstruction of capacitor 50, an electrical contact pin 68 offeedthrough 67 may contact cathode 53 to provide electrical access tocathode 53. In order to aid such a construction, tantalum encasementshells 55,56 may define U-shaped cross-sections in which shell 55 isdeeper than shell 56, and includes feedthrough 67 in proper alignmentwith cathode 53. If desired, a laser weld or solder may be used tocouple encasement shells 55,56 to one another where they abut oneanother.

Capacitor 50 is then sealed (75). Sealing capacitor 50 may comprisefilling any voids in capacitor 50 with a sulfuric acid/glycol baseelectrolyte, e.g., through a fillport formed in one of shells 55,56. Theelectrolyte can be aged and possibly cured to form and repair dielectricmaterial in capacitor 50. In addition, sealing capacitor 50 may furthercomprise re-filling capacitor 50 with electrolyte, and sealing afillport used to inject the electrolyte, thereby forming a hermeticallysealed capacitor 50. If desired, a substantially non-permeable film canalso be formed over capacitor 50 to improve the hermetic barrier to theinterior of capacitor 50. In that case, anode pins 65,66 and cathode pin68 may pass through the substantially non-permeable film. Alternatively,capacitor 50 may be packaged in a foil pack, an insulated metal case, orany other packaging material.

Capacitor 50 may provide one or more advantages, particularly when usedin an IMD similar to that illustrated in FIGS. 1 and 2. In particular,capacitor 50 may achieve reduced volume relative to conventionalcapacitor constructions. Alternatively, the same volume of capacitor 50relative to conventional designs may achieve improved energy storagecapacity. In particular, the efficiency of capacitor 50, i.e., thepercentage of volume used for energy storage may be greater than 70percent, greater than 80 percent, or even greater than 85 percent. Suchefficiency may be related to one of several factors, including the casepositive anode-cathode-anode design, reduced amounts of separatormaterial 61,62, reduced structural requirements of shells 55,56,increased structural integrity of anodes 51,52 and the specificmaterials used in the construction.

FIGS. 6 and 7 illustrate another embodiment of the invention in whichcapacitor 80 comprises a bi-polar case capacitor. In particular, FIG. 6is an exploded cross-sectional view of a capacitor 80, and FIG. 7 is anassembled cross-sectional view of capacitor 80. Capacitor 80 maycorrespond to either of capacitors 123,125 (FIG. 2). Capacitor 80comprises a single anode 81 formed in a manner similar to the formationof anodes 51,52, described above. For example, anode 81 can be formed bypressing tantalum powder into encasement shell 85, which may or may notform part of anode 81. Anode 81 may be in intimate contact withencasement shell 85. Encasement shell 85 may comprise a tantalumencasement shell having a U-shaped cross section. Optionally, a pressmay be used to form tunnels or holes in anode 81 prior to forming theoxide layer on the anodes 51,52.

Capacitor 80 also includes a single cathode 83 forming an opposing sideof the case structure. In particular, cathode 83 can be formed in amanner similar to the creation of cathode 53, described above, e.g., atitanium substrate can be coated on opposing sides of the titaniumsubstrate with hydrous ruthenium oxide, activated carbon, titaniumcarbide, etc. to form cathode 83. However, in this case, cathode 83 isnot sandwiched between anodes, but instead forms an opposing encasementshell of capacitor 80, i.e., the shell that opposes shell 85. Aseparator material 84 can be sandwiched between anode 81 and cathode 83.Moreover, at the interface of encasement shell 85 and cathode 83 aninsulative material 86 is used to electrically isolate anode 81 fromcathode 83. A laser weld or solder may be used to couple encasementshell 85 and cathode 83 to insulative material 86. Electrical contact toanode 81 and cathode 83 may be made via contact with the outer surfacesof capacitor 80.

FIG. 8 is a flow diagram illustrating creation of capacitor 80 having abipolar case capacitor design. As shown in FIG. 8, a tantalum powder ispressed into tantalum encasement shell 85 to define anode 81 (91). Insome cases, the press used to create anode 81 creates a series oftunnels or holes in anode 81 in order to aid in heat dissipation duringthe forming process and reduce ESR during operation. Also, one or morestructural enhancing elements may be provided within the tantalum powderduring pressing, in order to give anode 81 increased structuralintegrity. Again, similar to FIG. 5, FIG. 8 is devoid of specificillustration regarding the sintering of the anodes. As earlier stated,those of skill in the art will readily appreciate the necessarytemperature and pressure settings for appropriate sintering. Notably,however, as earlier described and herein claims, the sintering occurs toboth the pressed tantalum powder anode and the enclosure therefore. Atantalum pentoxide film is then formed over surfaces 87 to define theanode dielectric for anode 81 (92).

A titanium encasement shell is coated with hydrous ruthenium oxide toform cathode 83 (93). Encasement shell 85 and cathode 83, which formsthe opposing encasement shell, are then attached to one another (94).Insulation 86 is provided at the interface of encasement shell 85 andcathode 83 for electrical isolation of cathode 83 and anode 81.Separator material 84 may also be provided to electrically isolatecathode 83 from anode 81 in the final construction of capacitor 80.

Capacitor 80 is then sealed (95). Sealing capacitor 80 may comprisefilling any voids in capacitor 80 with a sulfuric acid/glycol baseelectrolyte, e.g., through a fillport formed in one of shell 85 andcathode 83. The electrolyte can be aged or cured to form and repairdielectric material in capacitor 80. In addition, sealing capacitor 80may further comprise re-filling capacitor 80 with electrolyte, andsealing a fillport used to inject the electrolyte in order to form ahermetically sealed capacitor 80. If desired, capacitor 50 may befurther packaged in a film, foil, metal case, or any other packagingmaterial.

A number of capacitor designs for use in medical devices have beendescribed. The capacitors may find application any of a wide variety ofmedical devices, but are most useful in applications where size is aconcern. Nevertheless, various modifications may be made withoutdeparting from the scope of the following claims. For example, thecapacitors may be used in numerous other devices, and may bespecifically desirable for applications where small-volume, high voltagecapacitors are desired. Non-medical device applications are alsoenvisioned. These and other embodiments are within the scope of thefollowing claims.

1. A capacitor, comprising: a first anode formed within a firstencasement shell, the first encasement shell having a first peripheraledge; a second anode formed within a second encasement shell, the secondencasement shell having a second peripheral edge; and a cathodesandwiched between the first and second anodes and within the first andsecond encasement shells, wherein the first encasement shell is deeperthan the second encasement shell such that when the cathode issandwiched between the first and second anodes and within the first andsecond encasement shells, the first encasement shell overlaps the secondanode and the first peripheral edge abuts against the second peripheraledge.
 2. A capacitor according to claim 1, further comprising separatormaterial sandwiched between the first and second anodes and within thefirst and second encasement shells to electrically separate the cathodefrom the anodes.
 3. A capacitor according to claim 1, furthercomprising: a first electrical anode pin formed through the firstencasement shell for electrical contact with the first anode; a secondelectrical anode pin formed through the second encasement shell forelectrical contact with the second anode; and a feedthrough elementformed through the first encasement shell, the feedthrough elementincluding a cathode pin within an insulative material for electricalcontact to the cathode.
 4. A capacitor according to claim 1, wherein theanodes provide more structural support to the capacitor than theencasement shells.
 5. A capacitor according to claim 4, furthercomprising one or more structural enhancing elements within the anodes.6. A capacitor according to claim 1, wherein the encasement shellscomprise tantalum and the anodes comprise tantalum powder pressed withinthe encasement shells and formed with a film of tantalum pentoxide onsurfaces of the respective anodes.
 7. A capacitor according to claim 6,wherein the cathode comprises a titanium substrate coated on opposingsides of the titanium substrate with hydrous ruthenium oxide.
 8. Acapacitor according to claim 1, further comprising one or more holesformed in the anodes.
 9. A capacitor according to claim 1, furthercomprising a substantially non-permeable film formed over the capacitor,wherein electrical contacts to the anodes and cathode pass through thesubstantially non-permeable film.
 10. A capacitor, comprising: an anodemechanically coupled to and electrochemically anodized within a firstencasement shell, the first encasement shell having first peripheraledge; a cathode forming a second encasement shell, the second encasementshell having a second peripheral edge; and insulative material along aninterface of the first and second peripheral edges to electricallyisolate the anode from the cathode.
 11. A capacitor according to claim10, the capacitor further comprising a separator material sandwichedbetween the first and second encasement shells to electrically separatethe cathode from the anode.
 12. A capacitor according to claim 10,further comprising one or more structural enhancing elements within theanode.
 13. A capacitor according to claim 10, wherein the firstencasement shell comprises tantalum and the anode comprise tantalumpowder pressed within the first encasement shell and formed with a filmof tantalum pentoxide on a surface of the anode.
 14. A capacitoraccording to claim 13, wherein the second encasement shell comprises atitanium substrate coated on opposing sides of the titanium substratewith hydrous ruthenium oxide.
 15. A method comprising: pressing tantalumpowder into first and second tantalum corresponding encasement shells todefine a first anode slug and a second anode slug, wherein the firstencasement shell is deeper than the second encasement shell and firstand second encasement shells have first and second peripheral edges;sintering the first anode slug and the second anode slug at elevatedtemperature in a pressurized chamber; immersing the first anode slug andthe second anode slug in a formation electrolyte; applying electricalpotential to the immersed first anode slug and the second anode sluguntil a desired amount of oxide grows on the respective surfacesthereof; coating a titanium substrate with a cathode active material toform a working cathode; wrapping the working cathode with at least onelayer of a separator material; sandwiching the cathode between the firstand second anodes; abutting first and second peripheral edges to form aninterface; and sealing corresponding edges of the encasement shells atthe interface.
 16. A method according to claim 15, further comprisingforming tantalum pentoxide films over the pressed tantalum powder todefine anode dielectrics for the first and second anodes.
 17. A methodaccording to claim 15, wherein the first and second tantalum encasementshells define cross-sectional depths that are different such that whenthe cathode is sandwiched between the first and second anodes the firsttantalum encasement shell overlaps with the second anode.
 18. A methodaccording to claim 15, further comprising forming one of apertures,bores, and ports in the tantalum powder.
 19. A method according to claim15, further comprising sandwiching separator material between the firstand second anodes to electrically isolate the cathode from the first andsecond anodes.
 20. A method according to claim 15, further comprisingpressing the tantalum powder with one or more structural enhancingelements into the first and second tantalum encasement shells.
 21. Amethod according to claim 15, further comprising sandwiching separatormaterial between the anode and the cathode.
 22. A method according toclaim 15, further comprising pressing the tantalum powder with one ormore structural enhancing elements into the tantalum encasement shell.23. A method, comprising: pressing tantalum powder into a tantalumencasement shell to define an anode; sintering the pressed tantalumpowder and the tantalum encasement shell at elevated temperature in anpressurized chamber; anodizing the pressed tantalum powder and thetantalum encasement shell in a formation electrolyte to form a layer oftantalum pentoxide over the exposed surfaces of the pressed tantalumpowder and the tantalum encasement shell to form a working anode slug;removing at least a majority of tantalum pentoxide from the peripheraledge of the titanium encasement shell; and abutting the peripheral edgesof the encasement shell of the working anode slug to a correspondingperipheral edge of a vessel housing a cathode, wherein a layer ofelectrical insulation is disposed between the respective peripheraledges.
 24. A method according to claim 23, further comprising formingtantalum pentoxide films over the pressed tantalum powder to define ananode dielectric for the anode.
 25. A method according to claim 23,further comprising pressing holes in the tantalum powder.